This invention relates to a hydrodynamic bearing assembly incorporated with a spindle motor used for driving a memory device such as a hard disk drive (referred to as a xe2x80x9cHDDxe2x80x9d, hereinafter), or a bar code scan reader, and in particular, relates to the hydrodynamic bearing assembly, which has an improved activation feature.
The hydrodynamic bearing assembly for use in a spindle motor of the memory device such as the HDD and a drive unit for driving a polygonal mirror in the bar code reader has been required to attain a high rate, stable rotation under a high load, to have a high bearing rigidity that prevents the rotational member from making contacts with the stationary member even under the existence of external vibrations, thereby to have a reduced starting torque, and to have an improved activation feature with a reduced wear caused by the frictional rotations.
FIG. 8 illustrates one example of a conventional spindle motor. In the drawing, a column shaft 1 and a disk-shaped thrust plate 4 are secured on a base 10. The thrust plate 4 is attached perpendicularly to the shaft 1. The shaft includes an outer surface parallel to the axis of the shaft 1. A cylindrical hollow sleeve 3 is rotatably arranged around the outer surface of the shaft 1 with a predetermined gap so that a hydrodynamic,bearing is defined between the shaft 1 and sleeve 3. Thus, the radial bearing is defined between the outer surface of the shaft 1 and the inner surface of the sleeve 3 for generating a radial dynamic pressure in the radial direction perpendicular to the axis. Also, a thrust bearing is defined between a bottom surface of the sleeve 3 (which is referred to as a thrust-opposing surface of the sleeve 3, hereinafter) and the thrust plate 4 for generating a thrust dynamic pressure in the thrust direction parallel to the axis. Grooves 5 for generating the thrust dynamic pressure are formed on a surface of the thrust plate 4 opposing to the thrust-opposing surface of the sleeve 3. A rotor 17 is attached with the sleeve 3 such that it can rotate together with the shaft 1 around the sleeve 3. Information media (in case of the HDD) or a polygonal mirror (in case of the bar code scan reader) is mounted on the outer surface of the rotor 17. A rotor magnet 18 is attached on the inner surface of the rotor 17 and opposes to a stator coil 19 mounted on a base 10.
FIG. 9 illustrates a detail of the grooves 5 formed on the thrust plate 4 for generating the thrust dynamic pressure. A plurality of spiral grooves is formed on the thrust plate 4, and each groove is designed to be inclined with a predetermined angle relative to the circle and generally has a width of several microns (approximately 1 to 5 microns). Although FIG. 9 shows the grooves 5 formed on the thrust plate 4, the grooves 5 may be formed on the thrust-opposing surface 13.
In the rotation of the spindle motor so constructed, the stator coil (not shown) energized by the electric flow generates the attraction/repulsion force. This provides a rotation driving force with the rotor 17 having the rotor magnet 18 to rotate both of the rotor 17 and the sleeve 3 secured thereto around the shaft 1. A relative movement between the shaft 1 and the sleeve 3 due to the rotation generates the radial dynamic pressure through a fluid such as air intervened therebetween. Also, the relative movement between the thrust plate 4 and thrust-opposing surface 13 of the sleeve 3 in cooperation of the grooves 5 generates the thrust dynamic pressure. The radial and thrust dynamic pressures keep the rotational member such as the sleeve 3 and the rotor 17 away from the stationary member such as shaft 1 and the thrust plate 4 during the rotation.
FIG. 10 is an enlarged perspective view of the hydrodynamic bearing assembly in isolation used for the spindle motor of FIG. 8. In the drawing, the thrust plate 4 is secured on one end of the shaft 1 perpendicular to the axis. The sleeve 3 indicated by a phantom line is rotatably arranged around the outer surface of the shaft 1. When the spindle motor is energized to activate, the rotational member such as sleeve 3 starts to rotate in contact with the thrust plate 4 due to its own weight. The spiral grooves 5 in cooperation with the rotation of the sleeve 3 indicated by the arrow 6 conducts the fluid such as air into the thrust bearing between the sleeve 3 and the thrust plate 3 and forces the fluid towards the center of the thrust plate 4 along the direction indicated by the arrow 7. A land portion 9 is defined between the spiral grooves 5 and the outer surface of the shaft 1, in which the forced fluid are compressed between the land portions 9 and inner end portions of the grooves 5 so as to generate the dynamic pressure for supporting the sleeve 3. Thus, according to the conventional hydrodynamic bearing assembly, the thrust dynamic pressure to be generated has peaks localized adjacent to the land portion 9.
FIG. 10 shows another example of the hydrodynamic bearing assembly, having the shaft 1 with another grooves 2 offset to the axis, which are formed on the outer surface and opposes to the inner surfaces of the sleeve 3. The grooves 2 are not essential to generate the radial dynamic pressure. However, the rotational member such as the sleeve 3 in the drawing rotates around the bearing axis of the stationary member such as the shaft 1, and also it may whirl (revolve) around another axis offset to the bearing axis, which is referred to as a half-whirl phenomenon. The half-whirl results in whirling of the functional components such as the information media and the polygonal mirror mounted on the rotor 17, thereby to cause malfunctions in utilizing the components. The grooves 2 formed on the outer surface of the shaft 1 advantageously avoid the half-whirl. The grooves may have various configurations for avoiding the half-whirl, including the offset grooves as shown in the drawing, grooves parallel to the axis, and the herringbone-shaped grooves. However, when the grooves 2 are offset to the axis, advantageously, the rotation of inner surface of the sleeve 3 in the direction indicated by the arrow 8 forces the fluid from the top end to the bottom end due to its viscosity, thereby further increasing the dynamic pressure in the thrust bearing. Also, the grooves 2 may be formed on the inner surface of the sleeve 3, rather than on the outer surface of the shaft 1.
The dynamic pressure distribution in the radial bearing is generated similar to that of the thrust bearing. Thus, the rotation of the sleeve 3 in the direction opposing to the grooves as indicated by the arrow 8, in cooperation with its viscosity, forces the fluid in the grooves from the upper end (right side of the drawing) to the lower end (left side). To this end, it is assumed that the dynamic pressure distribution is uneven, increasing the dynamic pressure adjacent the bottom ends of the groove 2.
FIG. 11 illustrates the hydrodynamic bearing assembly having the sleeve 3, which is whirled and inclined relative to the shaft 1 and the thrust plate 4 due to the external factors applied to the bearing assembly. The sleeve 3 is inclined counterclockwise relative to the shaft 1, the shaft 1 moves closer to the sleeve 3 at the upper right portion A and the lower left portion B in the drawing. Also, the thrust plate 4 moves closer to the thrust-opposing surface at the leftmost portion C.
The parallel lines indicated in the drawing schematically illustrates the dynamic pressure in the radial and thrust bearings when the shaft 1 is inclined relative to the sleeve 3. As the shaft 1 moves closer to the sleeve 3 adjacent to the portion A, the wedge effect due to the convolution of the fluid therebetween generates the higher dynamic pressure. The same effect is observed adjacent to the portion B. Therefore, the counter forces due to the dynamic pressure adjacent to the portions A and B are generated against the contacting force between the shaft 1 and sleeve 3, and the contact between the shaft 1 and the sleeve 3 is avoided unless the external oscillation force overcomes the counter forces.
Meanwhile, the fluid is guided from the circumference of thrust plate 4 towards the axis so that the dynamic pressure between the thrust plate 4 and the thrust-opposing surface 13 is lower adjacent to the portion C and greater towards the bearing axis. Thus, the dynamic pressure around the portion C is low even if the thrust plate 4 and the thrust-opposing surface 13 moves closer. This may cause the thrust plate 4 and the thrust-opposing surface 13 to contact with each other around the portion C, when the external force such as the oscillating motion is applied. Once they contact with each other, the friction force therebetween results the unstable rotation of the rotational member. Further, the rebound followed by the contact causes the undesired impact, which could bring the malfunction of the magnetic head used for the HDD, or could result an extensive damage to the spindle motor.
As can be seen from the above description, the conventional hydrodynamic bearing assembly has following several disadvantages. Firstly, the high dynamic pressure distribution has peaks localized in certain portions within the gap defined by the hydrodynamic bearings, and if the compressed fluid is gas such as air, then the air locally compressed in the portions may generate a dew in the portions, because the water vapor in the air is compressed. The dew may cause no adverse effect while the bearing assembly keeps rotating so that the continuous flow of the fluid blows off the dew. However, when the electric power is interrupted and the rotation is halted, the sleeve 3 stops and contacts with the thrust plate 4 while the dew is remained therebetween. To this end, the dew between the sleeve 3 and the thrust plate 4 causes them to closely fit with each other, resulting some activating disadvantages when the bearing assembly is restated.
Secondary, when the external forces is applied to the bearing assembly to tilt the shaft relative to the sleeve, the thrust bearing generates the dynamic pressure (particularly in the circumference thereof) insufficiently to bear the external forces. Thus, the sleeve 3 and the thrust plate 4 are likely to contact with each other due to such external forces. This comes from the fact that the dynamic pressure distribution has the peak in the portions adjacent to the bearing axis.
Thirdly, when the spindle motor is halted, the weight of the rotational member causes the thrust opposing surface 13 as the bottom surface of the sleeve 3 to contact with the thrust plate 4. Both of the thrust opposing surface 13 and the sleeve 3 have even surfaces allowing a full contact therebetween. When the spindle motor is restated, the greater activating torque, is required enough to overcome the friction force of the full contact. Thus, the capability of the motor should be increased, resulting more energy consumption. Further, the rotational member rotates relative to and in contact with stationary member until the dynamic pressure generated therebetween is enough for floating the rotational member away from the stationary member. The rotation in contact causes both members to wear and generate the abraded particles, which brings an adverse effect to the precise bearing assembly. Even worse, the seizure of the bearings may be caused due to the greater friction force. Thus, the rotation in contact reduces the endurance and the reliability of the hydrodynamic bearing assembly or the spindle motor incorporating thereof.
The conventional techniques have proposed various approaches in order to address those disadvantages. For example, Japanese Patent Laid-Open Publication Nos. 11-18357 and 11-55918 disclose, in particular, the technique for preventing the contact in the thrust bearing due to the tilt of the shaft. According to the prior art techniques, the coil is arranged eccentrically with the rotor magnet so that the shaft is biased against the sleeve in a predetermined direction, allowing the rotation in a stable manner. However, the art requires the coil to be positioned concentrically with the bearing assembly for biasing the shaft to the sleeve in parallel. This arrangement is often impossible because of the design restriction.
Japanese Utility Model Laid-Open Publication No. 55-36456 discloses the stationary permanent magnet attached to the housing opposing to the rotor magnet for tilting the rotor towards the predetermined direction for the rotation. However, this technique decreases the lifetime of the bearing assembly because the distal edge of the shaft contacts with the sleeve.
Also, Japanese Patent Laid-Open Publication No. 60-234120 discloses the technique for reducing the torque when activated, in which at least one of the thrust opposing surface and the thrust plate has a convex configuration in the thrust bearing as illustrated in FIG. 12. In the drawing, a plurality of grooves 32 for generating the dynamic pressure are formed on the thrust plate 31, and the shaft 33 rotates around the axis in a direction indicated by the arrow w. A thrust member 34 is secured on the bottom end, which opposes to the thrust plate 31. The thrust member 34 and the thrust plate 31 together define the thrust bearing. The thrust member is designed such that it has a spherical surface 35 with a predetermined radius R opposing to the thrust plate 31 and the spherical surface 35 protrudes by the protruding thickness N.
The rotation keeps the thrust plate 31 and thrust member 34 of the thrust bearing away from each other, and the halt of the rotation causes them in a small region. However, since the contacting area is a pinpoint, the rotational member can be activated and floated without any excessive torque nor galling.
However, the central portion of the spherical surface 35 may contact with the thrust plate 31 and the rotational member may not float away therefrom depending upon the radius R and the protrusion N of the spherical surface 35. Also, the thrust member has to be processed such that it has the spherical surface 35, in which it is difficult to form such a small convex surface.
Also, Japanese Patent Laid-Open Publication No. 9-328381 discloses the technique for reducing the frictional coefficient between the thrust contacting surfaces by forming one of the thrust contacting surfaces of an amorphous hard carbon film and the other of ceramics material having the void occupying rate of 6% or less and the maximum diameter of 10 microns or less.
However, while the reduced torque corresponding to the reduced frictional coefficient due to the fixed lubricant film is observed, no further improvement can be expected by this technique, because the thrust contacting surfaces are kept in full contact therebetween, the circumference thereof, in particular, has the longer radius, which can be harmful to contribute the reduction of the activation torque.
This invention is to provide a hydrodynamic bearing assembly, which eliminates the disadvantages of the conventional technique, realizes an even dynamic pressure distribution without extreme pressure peaks localized in the grooves for generating dynamic pressure to avoid the malfunctions because of the dew in the hydrodynamic bearings, and endures against the external oscillation applied to the thrust bearing. Also, this invention includes a process for preventing the dew from being generated in the hydrodynamic bearings by causing the dynamic pressure distribution in the grooves to be kept substantially even.
Further, this invention is to provide a hydrodynamic bearing assembly, in which the full contact between the thrust plate and the thrust opposing surface is avoided so that the friction and the energy consumption are reduced in comparison with the prior art technique, a reduced activating torque ensures the rotational member to float away from the stationary member, and the high rigidity against the external oscillation is achieved.
Even further, this invention is to provide a spindle motor rotating in a stable manner with an improved activation feature and the tilt rigidity as described, and also to provide a memory device and a bar code scanning device with the improved endurance and the reliability.
In particular, one aspect of the present invention is to provide the hydrodynamic bearing assembly comprises: a rotational and stationary members arranged with predetermined gaps to each other, the gaps including a radial gap defined therebetween in a radial direction perpendicular to an axis or a thrust gap defined therebetween in a thrust direction parallel to the axis, the gap containing a fluid for generating a radial or thrust dynamic pressure due to the relative rotation between the rotational and stationary members so that the rotational member rotate relative to the stationary member without any contact; wherein said rotational and stationary members have opposing surfaces to each other, and any one of the opposing surfaces has grooves formed thereon, the grooves having the depth modified in accordance with its position so that the dynamic pressure generated across the gap is substantially even. The depth of the grooves is modified to level the dynamic pressure so that the counter force against the tilting force in the thrust bearing is improved, and the dew generated under the peak pressure in the thrust bearing can be reduced. In order to level the dynamic pressure in the grooves, preferably, the grooves have the depth modified gradually and smoothly towards the downstream flow of the fluid passing therethrough.
Another aspect of the present invention is to provide the hydrodynamic bearing assembly comprises: a disk-shaped thrust plate extending in a radial direction perpendicular to a bearing axis; a circular thrust opposing surface extending in the radial direction and opposing to the thrust plate; and a thrust bearing for generating the thrust dynamic pressure in a thrust direction parallel to the bearing axis due to a relative rotation between the thrust plate and the thrust opposing surface; wherein at least one, or both of opposing surfaces of the thrust plate and the thrust opposing surface are inclined such that a distance between both opposing surfaces thereof becomes greater from an inside portion towards an outer portion of the thrust bearing. The gradient is preferably formed such that it hardly has an influence to the thrust dynamic pressure but avoids the contact due to the tilt. In particular, preferably, either one or both of opposing surfaces in the thrust bearing have the frustum or spherical configuration, and the distance difference of the gradients are within approximately 2 microns or less.
Further another aspect of the present invention is to provide the hydrodynamic bearing assembly comprises: a radial bearing including a column shaft having an outer surface parallel to an axis, and a hollow cylindrical sleeve having an inner surface rotatably arranged around the outer surface of the shaft, the iradial bearing for generating a radial dynamic pressure due to a relative rotation between the sleeve and the shaft; a thrust bearings including a thrust plate formed or secured on one end surface of the shaft along the axis, and a thrust opposing surface formed or secured on one end surface of the sleeve along the axis, the thrust bearing for generating a thrust dynamic pressure due to the relative rotation between the thrust plate and the thrust opposing surface; and a second thrust plate covering the hollow cylindrical sleeve at the other end surface along the axis; wherein a first gap a defined parallel to the axis between the other end surface of the shaft and the second thrust plate, and a second gap b defined parallel to the axis between the thrust plate and thrust opposing surface satisfy the following condition;
xe2x80x83a less than b.
The second thrust plate is provided so that the total weight of the rotational member is supported between the second thrust plate and the shaft when the bearing assembly is halted. This reduces the arm length of the friction when restarting (activating) the bearing assembly, thus eventually improving the activation feature of the hydrodynamic bearing assembly. Preferably any one of the second thrust plate and the other end surface of the shaft has a spherical, cone, or frustum boss, because the contacting points between the second thrust plate and the shaft can be closer to the axis.
Further another aspect of the present invention is to provide the hydrodynamic bearing assembly, in which one or more of opposing portions among the shaft, the sleeve, the thrust plate, and the thrust opposing surface, and the second thrust plate are made of ceramics material. The ceramics material is selected from a group consisting of alumina, zirconia, silicon carbide, silicon nitride, and sialon. Usage of the ceramics material having high anti-abrasion reduces the friction during the rotation with contact and provide the hydrodynamic bearing assembly having the high rigidity and the high accuracy.
Further another aspect of the present invention relates to provide a spindle motor incorporating the hydrodynamic bearing assembly having the improved activating feature and the high rigidity as described above, as well as to provide a memory device or a bar code scan reader incorporating the spindle motor. This invention is to provide such products having the high stability, the high reliability, and the high endurance.